guest blogger john hare
Many of my schemes bring to mind an old Far Side cartoon. Two filthy, scrawny convicts are hanging from their chains halfway up the dungeon wall. One of them says, “Now here’s my plan”. This post is another of the plans for the future that requires multiple steps before serious consideration. Like getting out of these gravity chains. Yesterday a very small team (Armadillo) demonstrated something that would have taken massive efforts to do just a few decades back. It is likely that a few decades from now something that would need a major world effort today can be accomplished by a focused team with private money.
In the post on moving asteroids, I made a number of assumptions not stated. One is that we have flown enough missions to the various targets to understand their composition, masses and movements. Another was that we were going to harvest the asteroid in the process of safing its’ orbit. That assumption sent me in the direction of throwing useful size payloads on Earth intercept trajectory during the infrequent launch windows. I considered a thousand ton payload to be about right. The comments suggested either a very few payloads much larger, or much smaller thrown masses for propulsion purposes only. Both ways can easily be superior to the original thought.
I assumed sending the asteroid material on a trajectory that would have it approaching the moon from ‘behind’ it’s velocity Earth relative. That would reduce the intercept velocity by around a kilometer per second. With relative approach velocities down to a km/sec or so, a skimming flyby would have a velocity only ~200m/s above Lunar escape velocity. A 200m/s impulse could be delivered by several means to the asteroid material at perilune to capture it into an eccentric Lunar orbit.
If there were volatiles on the asteroid, then they could be properly arranged for a Lunar laser to vaporize as Leik Myrabo does with the lightcraft. While it would take a large laser system, it could be the same one that is used to launch rockets from the moon by heating LLOX. Depending on the composition of the volatiles and the power of the laser, 5-10% of the incoming material would be used for braking to orbit. This would optimize to fairly low incoming asteroid payloads, so the 1,000 ton units would be too large.
A second method of slowing the incoming mass down is with the ‘airless aerobraking’ method. A series of globes of Lunar volatiles are thrown into the path of the payload. The impact vaporizes them and the volatiles ‘bounce’ in a rocket type reaction. Lunar volatiles of about 4-5% of the mass of the incoming payload would slow it to an eccentric orbit. Those Lunar volatiles wouldn’t even have to reach lunar orbit, just incoming payload altitude. While I thought it was an original idea, this guy is more qualified and thought of it first. http://www.walthelm.net/inverted-aerobraking/main.htm It would require some study to find the optimum size for this type capture.
A third method that was brought up in comments that two bodies and a tether can do some tricks a solid body cannot. If two chunks of the asteroid are connected by a tether as they approach the moon, the center of mass and velocity will be in a different place than either of the two end masses.
The center of mass and center of velocity on closest approach are 200-300 m/s above escape velocity. However, the lower mass, if cut loose at the proper time, will be at below Lunar escape. The upper payload is the propellant price of capturing the lower mass. This clearly optimizes to the largest possible chunks that can be handled.
The proponents of much larger chunks implied several very strong points that I had missed in the original concept. One is that the moon is only going to be in the right orbital position for a few days at best. The 180 or so 1,000 ton payloads that I suggested per launch opportunity would either have to arrive very close together with very little time between captures and some tricky orbital mechanics getting there, or they would have to rendezvous en route to be captured in one shot. Tricky either way. If the captures were spread over the whole month as I suggested, many of them would be arriving in a head on Lunar intercept with closing velocities of 3km/sec instead of 1km/sec. Single larger chunks would avoid this problem and make for a cleaner mission.
Another point for the larger chunks is that the threatening NEO doesn’t require much DeltaV at all to depart from the asteroid on an Earth trajectory. Since we were looking at an asteroid threatening Earth in 15 years, it probably has several near encounters in the meantime. Changing the NEO orbit by as little as 20m/s could put it on an impact course during one of those near encounters. That 20m/s also applies to payloads sent from the NEO to Earth intercept. While 20m/s doesn’t seem like much, six months or five years later when it intercepts, it changes the material position by over 30,000 to over 300,000 kilometers during those times. Beanstalks of 20m/s tip speed mass perhaps 4% of those for the 100m/s I suggested. This implies a payload of 25 times as much for the same mass of tether. The impulse to the threatening NEO is five times that of each payload that I suggested. Do that five times and the NEO orbit is influenced as much as by the gravity tractor in the whole 15 years. 25,000 ton chunks in eccentric Lunar orbit would seem to be a good start on an orbital manufacturing economy.
On the other hand, the much smaller tether for propulsion only has some strong points also. Doug suggested using a clothesline only approach with the tether making a complete loop around a wheel at the tip. The baskets or bags remain attached to the tether at all times and are filled with loose material enpassant and emptied at the end points losing no manufactured material at all for propulsion. Since there is no intent of capturing the propellant materials, thrusting can be continuous with a much smaller tether mass. For propulsion this is quite elegant. I would probably change to a longer Earth manufactured tether for higher Isp and lower gravel collection requirements.
When you go to the smaller propulsion only tether, the question is, “What do we want to do with this rock long term?” I say capture it and use it. Of course the above methods get a little inadequate for a gigaton dino killer, so I suggest using the moon for a capture into retrograde orbit at Lunar altitude or greater. Avoiding Lunar impact in another orbit or two will require some thought.
With 15 years to refine the approach, it might be possible to set up a gravity turn with the moon to place the NEO in a cycler orbit so that it passes L4, the moon, and L5 every two weeks. The 2 km/sec to ‘stop in L4 or5 can be done with a currently feasible tether with no expended propulsion mass. Spacecraft from the Earth to the cycler use a Lunar gravity turn plus 300m/s of propulsion at perilune to reach the gigiton cycler. Spacecraft from the cycler to the Earth fly from the tether to a Lunar gravity turn to TEI with only station keeping propellant. A gigiton of material with 2 km/sec less V to reach. and 2.3 km/sec less to return than the Moon, really does sound like a serious start on orbital manufacturing.
The retrograde dino killer wolf can now be one of our sheep dogs against other NEO wolves that are trying to thin the population. Large masses can be thrown into the path of dangerous objects that have relatively little warning time. Escape from Earth gravity at retrocycler altitude is under a half a kilometer per second for a straight shot. Even this can be reduced if we will accept multi thousand ton rocks doing another Lunar gravity turn to convert that retrograde orbit back to an interplanetary trajectory. Even another gigaton dino killer will notice a dozen kilotons impacting at 10+km/sec. We would still need a year of warning, but that beats 15 years for a gravity tractor. And really beats our current oh crap, we know we are done capabilities.